In recent years, the design rule for LSI has been progressively adopting more micronized orders of sub-quarter micron, and as a result the shortening of the wavelength of the exposure light source has been compelled. Thus, the exposure light source has been shifted from g-line (436 nm) and i-line (365 nm) lights of the mercury lamp to KrF excimer lasers (248 nm) and ArF excimer lasers (193 nm), and furthermore the EUV light exposure using EUV (13.5 nm) is being studied.
In manufacturing semiconductor devices such as LSI and Super-LSI or in manufacturing a liquid crystal display board, a pattern is made by irradiating a light to a semiconductor wafer or an original plate for liquid crystal; however, if a dust adheres to a photomask for lithography (also simply referred to as “mask”) or a reticle (these are collectively referred to as “exposure original plate” herein below) used in this pattern creating stage, the dust absorbs light or bends it, causing deformation of a transferred pattern, roughened edges or black stains on a base, and leads to problems of damaged dimensions, poor quality, deformed appearance and the like.
For this reason, the above-mentioned lithography works are usually performed in a cleanroom, but even so, it is still difficult to keep the exposure original plate clean all the time; therefore, in general the light irradiation is carried out only after a surface of the exposure original plate is sheltered by a pellicle. In this way, the dust particle is prevented from directly adhering to the surface of the exposure original plate but is caught on the pellicle, and if, at the time of the lithography, the exposure light is focused on the pattern described on the exposure original plate the dust particle on the pellicle does not partake in the image transferring.
The basic structure of a pellicle is such that a pellicle membrane having a high transmittance against lights used in exposure light procedure is tensely adhered to an upper end face of a pellicle frame, and a gasket is adhered to a lower end face of the pellicle frame as an airtight member. For this airtight gasket an agglutinant layer is used in general. Also, the pellicle membrane is made of cellulose nitrate, cellulose acetate and a fluorine-containing polymer or the like which transmit well such lights that are used in light exposure (e.g., g-line [430 nm], i-line [365 nm], KrF excimer laser [248 nm], and ArF excimer laser [193 nm]). Further, of late, as a pellicle membrane for EUV exposure a use of a single crystal silicon membrane having an extra thin thickness such as 1 μm or smaller or in particular 10 nm-1 μm is being studied.
This pellicle is provided for the purpose of preventing dusts from adhering to the exposure original plate so that the pattern-containing region of the exposure original plate is to be isolated from the atmosphere outside the pellicle so as to be free of dust in the atmosphere. Then, to install the pellicle, the agglutinant layer of the pellicle is pressed against the exposure original plate for adhesion in a manner such that the pattern-containing region formed on the surface of the exposure original plate is fully enclosed inside the pellicle frame.
On this occasion, a closed space defined by the pellicle membrane, the pellicle frame and the exposure original plate is created, and since this closed space is an airtight space, when the atmospheric pressure changes, a differential pressure between the inside and outside of the closed space would work upon the pellicle membrane. Now, if the membrane is made of a resin such as fluorine-containing polymer, the pellicle frame would swell outwardly or inwardly. To prevent this a pressure adjustment hole may be made through a side bar of the pellicle frame and further a filter may be put to cover the hole so as to prevent dust from passing into the closed space.
In the case where conventional i-line [365 nm], g-line [436 nm], KrF excimer laser [248 nm], or ArF excimer laser [193 nm] is used, the exposure procedure is conducted in the atmosphere within a cleanroom and the filter is generally made of a resin such as PTFE. In IP Publication 1 teaches that the filter provided to a pressure adjustment hole for dust prevention can be made of a stainless steel or a ceramic as well as a resin such as PTFE; however, in practice, when the exposure procedure is conducted in the atmosphere, as will be shown in an example, the filter is often made of a resin such as PTFE for its low cost and simplicity, stainless steel and ceramic are scarcely used.
On the other hand, in the case of EUV exposure procedure, which is conducted in vacuum or under reduced-pressure atmosphere, vacuum drawing is carried out at the time when the mask is brought in the exposure apparatus or brought out from it. If an extremely thin silicon film having a thickness of 1 μm or smaller, or preferably between 20 nm and 1 μm, and having a high transmittance for EUV light is used as the pellicle membrane, it is feared that a stress caused by a slight change in surrounding pressure may break the silicon film, and to prevent this by mitigating such differential pressure between the inside and the outside of the closed space, a pressure adjustment hole is made through a frame side bar to communicate between the inside and the outside of the closed space whereby air is allowed to pass through the hole. Also, in order to prevent a dust from passing into the closed space, a filter is provided to cover the pressure adjustment hole.